How could we tell time if there were no watches or clocks anywhere in the world? The sun was probably the world's first "clock", except in the far north, where the Eskimos live. There, it's dark most of the winter, and light most of the summer. But inmost of the world, people have used clock. Even today, if you don't have a clock, you still know that when the sun shines, it's day, and when it's dark, it's night. The sun can also tell you if it's morning, noon, or afternoon.
People who live near the sea can tell time from the tides (潮汐). In the day time, for about six hours, the water rises higher and higher on the beach. And then it goes down and down for about six hours. The same thing happens again at night. There are two high tides and wo low tides every 24hours. Seamen on a ship learn how to tell time by looking at the moon and the stars. The whole sky is their clock. In some places in the world the wind comes up at about the same time every day or changes direction or stops blowing. In these places the wind can be the clock.
A sand clock is an even better clock. If you had fine dry sand in a glass shaped like the one in the picture, you would have what is called an hour glass. The sand in the hour glass goes from the top part to the bottom part in exactly an hour. When the hour glass is turned over, the sand will take another hour to go back again
